You can draw a personal pension or one from previous employer from 55 (will be 57 from 2026), even if still working.
But current employer won't pay both salary and pension simultaneously, you need to stop work and retire to get it.
What happens with most pensions is if you draw early (or late) the amount you get each month is adjusted, will be something like 97% per month if year early, 93% per month if 2 years early (not linear as it is ratio of expected payout) etc
Tax wise, can always have upto 25% as tax free cash lumpsum. Any other income will be taxed (at 20, 40 or 45% depending on other income).
There are rules about putting more money into pensions when you start drawing it (to stop you recycling part of tax free lumpsum and getting tax credit on new contributions). From memory limit is £4000 per year, so if you carry on working and pension contributions continue (from employer and/or employee) then must not exceed this (hefty penalty tax rates if you do). If your pension is salary related then need to be very wary of this, as bit of inflation can mean employer pays in quite a bit to cover next 20-25 years payout at higher rate. (roughly a £10 per year increase will require about £250 added to pension pot), so pension increase of anything over about £130 per year might exceed this recycling limit.
